This section is intended to provide a background or context to the invention that is, inter alia, recited in the claims. Unless otherwise indicated herein, what is described in this section is not prior art to the description and claims in this application and is not admitted to be prior art by inclusion in this section.
There is a need to provide clean, safe, and sustainable energy to supply the growing global demand for energy. In contrast to other renewable resources whose globally extractable power is each less than 5 TW, the solar constant upon the earth's surface (120,000 TW) will exceed worldwide energy demand into the foreseeable future. This abundant supply of sunlight and the environmental friendliness of solar energy make the efficient conversion of solar radiation into electricity a compelling economic and scientific goal.
Photovoltaic (PV) devices that covert solar energy to electricity have been in use for several decades. However, widespread adoption of PV devices has been somewhat limited by their expense and inefficiency relative to other sources of energy. Advances in structures and materials are important to achieving widespread adoption of effective PV devices. The most desirable materials for use in PV, applications are both inexpensive and abundant. In general, however, these materials have high impurity and defect concentrations, both of which limit the diffusion length of charge carriers which is a critical performance parameter of PV devices. The reduced length over which charges may be extracted (LCE) effectively limits the useful thickness of the active layer, as thicker layers will not increase device efficiency. For example, the efficiency of silicon PV devices scale with semiconductor quality as the LCE approaches the active layer thickness required to collect 90% of incident photons (LOD). Decoupling these two important parameters has recently been recognized in several PV fields, including polymer and Si p-n junctions. The initial experimental results in this direction are modest, owing to the complexity of fabricating such sophisticated geometries.
Structural improvements in PV devices has opened the door to the use of new, less expensive materials that often possess highly desirable characteristics. To this end, there has been interest in using certain transition metal compounds, including copper compounds, among others. For example, various attempts using Cu2S in PV devices have been made. The first thin film. PV devices comprised CdS/Cu2S heterojunctions formed through the topotaxial exchange of Cd atoms on a CdS surface for Cu+ ions in solution. Interest in the technology waned in the late 1980s, however, in light of the instability of the heterojunction through which Cu+ ions are prone to diffuse. In its place, the more costly ternary chalcogenide chalcopyrites (CuMX2) [M=In, Ga; X═S, Se] have been developed, with power efficiencies now approaching 20%.
More recently, it has been shown that it is feasible to form stable p/n heterojuctions with CuxS paired with an alternative n-type semiconductor, TiO2. As cation exchange is not possible in these Cd-free PVs, alternative deposition methods for CuxS are required. More specifically, chalcocite (CuxS, x˜2) is favored for PV applications among a sizeable number of copper-poor polymorphs (CuxS, 2≧x≧1.75) as it offers the largest spectral coverage and most appropriate conductivity. In addition to traditional topotaxial growth, previous routes to phase-pure chalcocite include single-source evaporation and reactive sputtering. Techniques using nanoparticle syntheses and aerosol assisted chemical vapor deposition (AACVD, a liquid precursor delivery technique akin to spray pyrolysis) have also been attempted.
Atomic layer deposition (ALD) is one technique for producing thin, conformal layers. The ALD of Cu2S has also been attempted by alternating exposure to Cu(thd)2 (thd=thd-2,2,6,6-tetramethyl-3,5-heptanedione) and H2S. However, existing methods for formation of Cu2S ALD are not adequate. For example, over the range of temperatures in which the deposition is self-limiting, the growth rate is slow, not exceeding 0.4 Å/cycle. Notably, only the less desirable CuS (covellite) and/or Cu1.8S (digenite) phases are obtained using this approach. Additionally, thermal decomposition of the Cu(thd)z precursor limits the ability to coat nanoporous supports desirable in advanced PV devices. CuCl2 can also be used, but this requires excessively high temperatures, and the HCl byproduct can result in etching of the films and the deposition equipment. Thus, a substantially pure (and highly desirable) chalcocite phase has not been achieved with these precursors.